Ceramics exhibit excellent characteristics in mechanical strength, heat resistance, abrasion resistance etc. but in general are joined with difficulty to metal. In particular, the strength at the joint is so low that the desired characteristic of ceramics may not be sufficiently displayed and that the subsequent application thereof is restricted.
As a method for joining ceramics and metal, especially as a method for treating the surface of ceramics before joining them, there are mentioned a molybdenum-manganese method, an active metal method, an oxide solder method and a high melting method etc.
In the molybdenum-manganese method, a fine powder which is added with Mo, Mo-Mn, W, W-Mn etc. is mixed with an organic binder to form a paint, the surface of ceramics is coated with the paint and metallized in a wet hydrogen or wet forming gas at a temperature of 1,300.degree.-1,700.degree. C., and then nickel-plated and joined to a metal by means of a solder. In this method, the Mn in the metallized layer is reacted with the moisture in the wet forming gas to form MnO, which is dissolved into the glass layer of ceramics and joins the ceramics to the metallized layer. The plated nickel formed on the surface of the metallized layer then enters into the layer during heat treatment thereafter or by soldering and diffusion in Mn and Mo resulting in a joint.
In the active metal method, an active metal such as Ti, Zr etc. is used, which is active at a high temperature and reacts easily with ceramics. Namely, an active metal in the form of plate or foil is placed between the ceramics and a metal to be joined and then heat-treated in vacuum or in an inert gas to adhere them. An active metal soldering method is described in NIKKEI MECHANICAL, Jan. 13, 1986. In this active metal soldering method, both a solder and an active metal are used wherein a solder in the form of foil is used together with an active metal or a solder added with an active metal is used. A method is also disclosed wherein an active metal such as Ni in the form of powder is applied onto the surface of ceramics such as Si.sub.3 N.sub.4 and heated under vacuum to metallize it.
The methods as mentioned above are the pretreatment methods through which the surface of ceramics is metallized to make it capable of being soldered, however the application of such a metallizing method still could not obtain a sufficient joint strength.
There are mentioned particular methods for joining ceramics and metal as follows:
As an active metal soldering method, there is mentioned a previous example in which a Ti-Cu-Ag alloy solder was used, whereby the high temperature strength on the joint boundary surface was low and the reduction of strength occurred at a temperature above 400.degree. C. (Zairyo Gijutsu, Vol. 4, No. 2, 1985).
Further, even in the method in which a metallized film is formed on the surface of ceramics to join ceramics and metal, a method has not been discussed, by which a metallized film may be formed, which exhibits good characteristics (high temperature strength) even at a temperature above 500.degree. C. In the method for forming a metallized film using an active metal, in which Ti is contained in a solder and a metallized surface is formed to join the ceramics and metal, the solder was softened at a high temperature, however, the strength was greatly reduced at this high temperature range, and therefore this method was not used, whereas an acceptable strength was obtained at a temperature from room temperature to 400.degree. C.
In the high melting metal method, a fine powder added with Mo, Mo-Mn, W-Mn etc. is mixed with an organic binder to form a paint, which is coated on the surface of ceramics, metallized in a wet hydrogen or a wet forming gas at a temperature 1,300.degree.-1,700.degree. C. and nickel-plated, and then the ceramics is joined by means of a solder to a metal. In this method, Mn in the metallized layer reacts with the moisture in the wet forming gas to form MnO, which is dissolved in the glass layer in the ceramics to join the ceramics and the metallized layer. The Ni-plating formed on the surface of the metallized layer then enters into the metallized layer during heat treatment or soldering to disperse in Mn and Mo with each other to join the ceramics and metal.
Moreover, an experiment is reported in Suganuma et al., Communication Amer. Ceramic Soc., (1983), c-117, in which ceramics of the nitride series and Mo or Fe were heated under a pressure of 3 GPa and a temperature of 1,300.degree.-1,400.degree. C. or more to join them. A method is also disclosed in which a mixed powder of Mo and Si.sub.3 N.sub.4 is placed as an intermediate layer to moderate remaining stress.
In Nicholas et al., J. Mater. Sci., 13 (1978), p. 712, a study is reported in which in order to search the effect of the remaining stress on the joint strength, aluminum is used as an insert to join various kinds of metals and Al.sub.2 O.sub.3.
However, by the above-mentioned methods for joining ceramics and metal, a sufficiently high joint strength between ceramics and metal could not be obtained. The previous technology for joining ceramics and metal provides only such a joined body that has a joint strength up to 15-20 kg/mm.sup.2, which does not reach to a level of 30 kg/mm.sup.2 required for the application to machinery parts, resulting in no development in the application of a ceramics-metal jointed body.
As a first consideration, problems will be described in the metallization method for forming the boundary surface between ceramics and metal. Among the previous methods as described above, the active metal method is advantageous in the formation of strong boundary surface of a joint. A previous method for metallizing ceramics by means of active metal, there has been proposed a method by means of an alloy solder mixed with an active metal or a method in which an active metal in the form of foil or plate is used. By these methods, however, an active metal of high purity can not be used, as a result of the reduction in activity due to the presence of impurities. On the other hand, the active metal is apt to be affected, for example, oxidized due to its high activity, which is not utilized efficiently. For example, though a method is proposed in which Ti of high purity is deposited on the surface of ceramics by the ion-plating method, the Ti-layer deposited is joined in the solid phase to the surface of ceramics so that it shows low wettability to ceramics and the reaction with the ceramics does not progress resulting in a low boundary strength.
The previous technique provides a joint boundary surface strength of 20 kg/mm.sup.2 and therefore a technique for the formation of boundary surface providing a high boundary surface strength has not been discovered.
For jointing a metallized ceramics and metal, there have been the following problems. One of the fundamental reasons that a high joint strength can not be obtained in the jointing of ceramics and metal is due to the fact that a great difference exists between the heat expansion coefficient of the ceramics and that of the metal, and that a large thermal stress during the jointing results in an impossible jointing or a great reduction of joint strength. The heat expansion coefficient of ceramics (in general) is within the range of 3.8-8.0.times.10.sup.-6 /.degree. C., whereas the heat expansion coefficient of metal is 10-20.times.10.sup.-6 /.degree. C. The difference between these expansion coefficients causes a large thermal stress between the ceramics and the metal on heating or cooling during the joining or its use. Such a problem has not been solved by any one of the above-mentioned methods.
Further, as a method for joining mechanically a ceramics part and a metal part, methods for casting, sintering, inserting, bolting and so on have been previously used. However, these methods require complex designs and applications and occasionally result in an unevenness and the breaking of ceramics owing to the thermal stress during the joining step (Kogyo Zairyo, Vol. 31, No. 12, p. 51). Moreover, these methods show less resistance to thermal shock and the like and have not been widely used in practice.